The present invention relates to an ionization-type smoke detector and, more particularly, to a heat-resistant case which prevents the scattering of radioactive source in an ionization-type smoke detector, and to the method of forming such a case.
An ionization-type smoke detector detects changes in ionization current due to smoke, etc., and since for this purpose it is provided with a radioactive source to ionize the air within the inner and outer ionization chambers, if the detector is destroyed by high temperatures at the time of a fire, there is the danger that the radioactive source may scatter. Therefore, the detector needs to be covered with a heat-resistant case. Since each of the electrodes of an ionization-type smoke detector are made of metal, the heat-resistant case can be economically constituted by utilizing the case as the outer electrode and by forming the rear cover as a metal plate.
Now an example of an ionization-type smoke detector of this kind will be explained in reference to FIG. 1 of the accompanying drawings where reference 1 denotes an insulating base board and below its undersurface are supported an inner electrode 4 mounted by means of a screw 3 with a radioactive source 2, an intermediate electrode 6 having a central hole 5, and also a net- or grid-like air permeable outer electrode 7 whereby inner and outer ionization chambers 8 and 9 are formed within the intermediate electrode separating both chambers. The outer electrode 7 covers the side surface of the insulating base board 1 up to its uppermost portion. The reference numeral 10 is a circuit board constituting a circuit to detect electrical changes in the outer ionization chamber 9, the electrodes 4, 6 and 7 being connected to the circuit board 10. Further, the reference number 11 is a rear cover made of a metal plate put on the rear side of the insulating base board 1 at its upper portion.
With such a construction, since the detector can be wholly covered with a heat-resistant case made of metal comprising the outer electrode 7 and the rear cover 11, even if the detector is destroyed at the time of a fire, etc. there is no danger of the radioactive source scattering. When constituting such a heat-resistant case, where the insulating base board 1 is circular the outer electrode 7 as a whole may be made in a cylindrical shape which can be easily formed by a drawing process. However, in a case where the insulating base board has a square shape, it becomes difficult to shape the outer electrode 7 entirely through a drawing process since at least the portion of the outer electrode 7 which surrounds the insulating base board 1 has to be shaped so as to have a square form.